1. Field of the Invention
The present invention concerns a method for controlling temperature in a patient undergoing ultrasound therapy under the guidance of magnetic resonance thermometry, and in particular to a rapidly convergent, multi-input, multi-output, non-parametric temperature controller for implementing such a method.
2. Description of the Prior Art
The localized application of ultrasound to treat certain types of local cancer often takes place with the patient being located, during the therapy, in a magnetic resonance imaging system, so that the temperature of the patient can be monitored by magnetic resonance thermometry. Magnetic resonance thermometry is a known manner of monitoring the temperature of an examination subject while the subject is located in a magnetic resonance data acquisition unit (scanner), and is based on the known phenomenon that the magnetic resonance signals emitted by nuclear spins in the examination subject, which have been forced out of a steady-state condition by the application of energy thereto, are temperature-dependent. The precessing nuclear spins that have been forced out of their equilibrium state emit signals that are detected and are used for conventional magnetic resonance imaging. The temperature-dependency of these signals also allows a temperature map of the examination subject to be generated, that very precisely shows temperature variations along multiple axes.
High intensity ultrasound is effective for use in treating localized cancers or other pathologies. For this purpose, the high intensity focused ultrasound (HIFU) is administered.
In order to steer and focus the therapeutic ultrasound, such an ultrasound head or device typically has multiple ultrasound emitters (transducers) arranged and operated in an array.
In general terms, magnetic resonance guiding or monitoring of such therapy takes place by administering HIFU to the patient in the examination region of a magnetic resonance scanner. Magnetic resonance data are acquired in a known manner from the patient in the scanner while the ultrasound therapy is in progress. In real-time with the therapy, magnetic resonance thermometry images of the patient are generated, showing the temperature distribution within a designated region of the patient. The resulting MR thermometry image can be shown on a monitor in real-time during the therapy for visual review and manual control of the therapy by a physician or a technician, or known image processing techniques can be used in order to generate appropriate extractions of information from the thermometry image for use in automatic control of the therapy.
An overview of MR-guided focused ultrasound hyperthermia is described in “Hyperthermia by MR-guided Focused Ultrasound: Accurate Temperature Control Based on Fast MRI and A Physical Model of Local Energy Deposition and Heat Conduction,” Salomir et al., Magnetic Resonance in Medicine, Vol. 43 (2000) pages 342-347. Various automatic control techniques are described, for example, in “Automatic Spatial and Temporal Temperature Control for MR-Guided Focused Ultrasound Using Fast 3D MR Thermometry and Multispiral Trajectory of the Focal Point,” Mougenot et al., Magnetic Resonance in Medicine, Vol. 52 (2004) pgs 1005-1015; “Three-Dimensional Spatial and Temporal Temperature Control with MR Thermometry-Guided Focused Ultrasound (MRgHIFU),” Mougenot et al., Magnetic Resonance in Medicine, Vol. 61 (2009) pgs 603-614 and “Curvilinear Transurethral Ultrasound Applicator for Selective Prostrate Thermal Therapy,” Ross et al., Medical Physics, Vol. 32, No. 6 (2005) pgs 1555-1565. A prevalent technique for temperature control in such therapy is to make use of a proportional-integral-derivative (PID) controller. In general terms, PID controller calculates an error value as the difference between a measured process variable and a desired reference point. As indicated by its name, a PID controller makes use of three feedback reaction terms, respectively referred to as proportional, integral and derivative values. The proportional value is dependent on the current error, the integral value represents an accumulation of past error, and the derivative value represents a prediction of future errors, based on a current rate of change. Typically, a weighted sum of these three values is used to adjust the process in question, by adjusting some type of control element that has the capability of changing the monitored parameter of the process in question. A study of the use of PID temperature control in the context of MRI-guided phased-array contact ultrasound is provided in “Endocavitary Thermal Therapy by MRI-guided Phased-Array Contact Ultrasound: Experimental and Numerical Studies on the Multi-input Single-Output PID Temperature Controller's Convergence and Stability,” Salomir et al., Medical Physics, Vol. 36, No. 10 (2009) pgs 4726-4741.
The PID-based controller in general exhibit good stability and robustness to noise, but its convergence is rather slow. When the number of temperature sampling points during the controlled sonication is low, the PID algorithm does not have sufficient time to adjust the power level so as to compensate for local variability in heat deposition or tissue cooling by diffusion/perfusion. The condition is worsened when the total time base is short, at the scale of the tissue response time to an elementary heat source. Moreover, when the static tuning of the physical parameters in the underlying model is far from the true parameters, overshooting of the temperature curves occurs with transient but significant and possibly longer heating above the prescribed temperature.